Devices to deliver fluids intravenously to a patient involve a number of considerations, such as air or gas bubble detection, gas removal, and flow rate control.
Air and bubble detection in medical intravenous (IV) fluid delivery systems is important. Large amounts of air can cause air embolisms in any part of the body, blocking off blood flow. Embolisms in the brain can cause severe memory loss and even death. Air trapped in the heart can also cause death or heart damage. Ultrasonic, optical, and electrical conductivity methods are used in the prior art detection of air bubbles in medical IV fluid lines.
Ultrasonic detectors are the most widely used detectors in the IV medical fluid delivery field and are based on the fact that sound is more readily conductive through liquid than through air. Thus, an air bubble does not “conduct” sound from one side of the tubing wall to the other, while fluid does conduct sound. Ultrasonic detectors are effective at detecting small amounts of gas in IV tubing, but have a number of drawbacks. They are expensive. They require that the tubing be in direct contact with the ultrasonic transmitter and receiver. Moreover, the slightest air gap can trigger the detector, causing a false alarm. Micro bubbles that build up on the surface of the tubing and are too small to be harmful also can trigger false alarms, since the micro bubbles, despite their very small size, still provide a boundary to the ultrasound. In addition, ultrasonic detectors have a fairly high power consumption, greater than 100 mW.
Optical detectors are typically inexpensive. Some optical detectors work using light absorption while others use light transmission. These methods are, however, fluid dependent and therefore not very common, because many different fluids are used for IVs. Also, their performance is dependent on the optical characteristics of the tubing, and many different tubing sets, having different optical characteristics, can be used. Additionally, optical detectors can be subject to interference from light from other sources.
Electrical conductivity detectors are used the least, as they require a direct electrical connection to the IV fluid. To electrically isolate the patient, this connection must have low leakage current and high dielectric strength. Typically, two or three electrodes are placed in contact with the fluid and are excited from an AC or DC source while the current/voltage is monitored. Gas bubbles do not conduct electricity, but many IV fluids do. A drawback, however, is that some IV fluids do not conduct electricity. Another drawback is that a thin film of fluid connecting one electrode to the other where the electrode penetrates the tubing wall can give a false detection of fluid presence.
When a fluid is heated, outgassing occurs. In prior art IV fluid warming devices, outgassing has either been ignored or handled with elaborate schemes. In one scheme, a hydrophobic filter has been employed to vent gases. This system is disadvantageous, because it is difficult and expensive to test to ensure that the filter does not leak. Also, the check valve used to prevent air from entering the system can stick, for example, if the humidity becomes too high or if another fluid inadvertently drips onto the valve, thereby requiring a greater pressure to open the valve. In another scheme, a drip chamber is used to collect the gases. This scheme is disadvantageous, because the chamber has a fixed volume and once full, the air can enter the patient unless a manually operated venting drip chamber is employed. A user must remember to vent such a venting drip chamber.
Intravenous (IV) fluids need to be delivered at different rates. Hydration fluids typically are delivered at higher rates, while drugs are typically delivered at lower rates. Flow rate control in medical IV fluid devices involves considerations of flow rate accuracy, errors made by personnel in setting flow rates, cost, and set up time. Three main types of devices are used in the control of IV fluid flow rates, namely, roller clamps, volumetric pumps, either volume displacement pumps or valve-regulated gravity assist pumps, and in line mechanical flow regulators.
Roller clamps are the most widely used flow control device. The roller clamp comprises a wheel trapped within a housing that compresses the IV tubing as it is slid along a gradual ramp. The flow rate is calculated by counting drips in a drip chamber. This device is inexpensive, but has a number of drawbacks. The setup operator must take time to count drips into the drip chamber, an iterative process taking up to 15 seconds for each adjustment. Also, the setup operator must know the size of the drips and must calculate the flow rate and may make a mistake. Even after it is set up, the IV tubing in the clamp continues to deform over time, causing the rate to change. The IV solution must be held above the patient insertion site. Any changes in height can affect the flow rate, because the roller clamp is a relative device. The advantages of the roller clamp are that it does not require any power, it is widely accepted, and it is inexpensive.
Volumetric pumps are also widely used for drug delivery. There are two main types of infusion rate control. In a first type of control, a displacement pump forces fluid through the IV line at repeatable volumes and adjustable intervals. These pumps can be reciprocating piston, peristaltic (linear or rotary), or syringe types. These pumps are typically quite precise, as required for drug delivery, and are not typically used for standard IVs. A second type of infusion rate control utilizes gravity driven fluid. With this type, drips through a drip chamber are counted, and a variable orifice valve is controlled based upon the number of drips over time.
A disadvantage of such pumps is their great expense. Also, the tubing set is typically disposable, which further increases the cost. These pumps also take up a lot of space. The main advantages of such pumps are accurate flow control, no change in flow with change in bag height, reduced setup time, and reduced chance of error by the operator.
In line mechanical flow regulators, using diaphragms, needles valves, and the like, are not very common. They are advantageous in that they require no power and are reasonably independent of IV fluid bag height. They are, however, dependent on fluid viscosity. Also, they typically have two flow rate scales (ml/min and ml/hour), which, while providing versatility, can also be confused by operating personnel.